Mixers (sometimes alternatively referred to as “blenders”) are generally employed to disperse powdered chemicals into fluids. One application for mixers is in wellbore operations, for example, in preparing hydraulic fracturing fluid for injection into a subterranean formation. Generally, the fracturing fluid includes gelling agents, powders and other granular material, e.g., guar gum, which are initially dispersed into the fluid via the mixer, and subsequently hydrated, e.g., in tanks, to result in the desired viscosity for the fluid.
Certain powder and granular material mixers include a centrifugal pump and eductor, or a centrifugal or high or low shear blender for dispersing the powder and granular material into fluid (e.g., water). Generally, the fluid is pumped by the pump into a mixing chamber. In eductor mixers, the mixing chamber may be proximal to a throat of a converging-diverging nozzle such that the eductor draws the powder into the mixing chamber by the Venturi effect. In blender mixers, the blender is located in the mixing chamber, and the powders and grains are fed thereto, e.g., by gravity. In either case, the materials, e.g., in the form of dry powder, are introduced to the mixing chamber, and are dispersed into the fluid. Various devices are employed to avoid air entrainment during the dispersion process, or entrained air may be removed downstream, e.g., using a hydro-cyclone or another type of air separator. The fluid mixture may then be sent to equipment downstream for further hydration.
One challenge in dispersing powder additives such as gelling agents is that the powders may tend to agglomerate into clumps, sometimes referred to as “fisheyes.” The powders may have cohesive properties, such that partially-hydrated balls form, e.g., with dry powder surrounded by a “skin” of partially-hydrated powder. This skin prevents hydration of the dry powder within, resulting in a stable fisheye in the fluid, rather than an even dispersion of the powder. As such, suboptimal mixing may result, which can affect downstream application. Moreover, there is an additional risk of buildup and/or clogging of the material, e.g., in the various throats of the system, if the materials are not sufficiently wetted at the point of introduction into the mixer.
Accordingly, in some instances, a pre-wetter may be employed to mitigate the risk of such clumping. Pre-wetters generally provide a fluid to the powder feed, upstream of the mixing. However, pre-wetters require a separate pump to deliver the fluid to the powder, upstream of the mixing chamber. Thus, additional pumping equipment (i.e., centrifugal pumps to provide fluid to pre-wetter) may complicate the overall system, adding costs, maintenance, and failure points. Moreover, the different pieces of equipment may limit the range of flowrates achievable for the system, limiting the applications for which a single size or configuration of mixer is suitable.